1. Field of the Invention
This invention relates to a novel polyorgano(hydro)silazane, which is useful as a ceramic starting material, a polymer hardening agent, a densifying agent, a surface coating agent, etc., and can be produced according to a process having a lower cost and a superior stability to that of polyorgano(hydro)silazanes of the prior art. The present invention also relates to a process for producing such a novel polyorgano(hydro)silazane.
2. Description of the Related Art
Known in the art are polyorgano(hydro)silazanes which are produced by allowing organodichlorosilanes such as methyldichlorosilane to react with dry ammonia in a non-reactive solvent. The polyorgano(hydro)silazanes produced by this method have a composition represented by --RSiHNH).sub.n [wherein R is an organo group such as an alkyl group, aryl group, etc., and n represents a polymerization degree], but the polymerization degree n thereof is low and the composition comprises a mixture of cyclic compounds with n=3 to 5.
It has been proposed to produce a polysilazane with a high molecular weight by using such a cyclic product with a low polymerization degree according to a polymerization of the cyclic product by ring opening with a heat treatment at 100.degree. C. to 300.degree. C. in the presence of a clay-like solid catalyst (Japanese Unexamined Pat. Publication (Kokai) No. 54-93100). Also, by using the same cyclic product with a low polymerization degree as mentioned above, it has been proposed to produce a polysilazane with a high molecular weight according to a polymerization of the cyclic product by mixing it with bis(trimethylsilyl)amine and subjecting it to ring opening by heating at 110.degree. C. in the presence of a ruthenium catalyst.
On the other hand, also known is a ladder-like polysilazane which is prepared by allowing an organo-dichlorosilane to react with dry ammonia, and then removing ammonium chloride to obtain a cyclic silazane with a low polymerization degree, followed by a further reaction with the use of a catalyst such as KH, NaH, etc., (D. Seyferth et al., Communication of the American Ceramic Society, C-132(6), 1984). The ladder-like polysilizane with a high molecular weight is represented by the following formula: EQU (R.sup.1 SiHNH).sub.a (R.sup.1 SiHNR.sup.2).sub.b (R.sub.1 SiN).sub.c
wherein R.sup.1 is an alkyl group, an aryl group, etc., R.sup.2 is hydrogen, an alkyl group, etc., and 0.37.ltoreq.a.ltoreq.0.41, 0.02.ltoreq.b.ltoreq.0.04, 0.57.ltoreq.c.ltoreq.0.60, a+b+c=1.
The above cyclic polysilazane with a low molecular weight is not desirable as a ceramic starting material because it is susceptible to hydrolysis with water, etc., in the air, and the vaporization loss during firing at a high temperature is great. Further, the polysilazane is produced at a disadvantageously low yield.
Also, the high molecular weight polysilazane polymerized by ring opening the low molecular weight cyclic polysilazane is a firm polymer insoluble in organic solvents and, therefore, it is not desirable for utilization as a ceramic starting material, a densifying agent, a polymer hardening agent, or a surface treating agent.
The ladder-like polysilazane with a high molecular weight as mentioned above, although it is of interest as a ceramic starting material, requires a large number of steps and expensive catalysts or reaction terminators such as potassium hydride or methyl iodide (which cannot also be reused) in its preparation. Besides, since it is required to highly dry tetrahydrofuran used as the solvent, the production cost per unit weight of ceramics is disadvantageously high. Also, while it is indispensable to use a catalyst such as potassium hydride and a solvent such as dry tetrahydrofuran in its production, potassium hydride will react vigorously with water and will ignite if an organic solvent is co-present. On the other hand, dry tetrahydrofuran will be readily oxidized to form a highly explosive peroxide. Thus, these compounds are disadvantageous in that they can be handled only with extreme difficulty and with a low safety factor.